1. FIELD OF THE INVENTION
The present invention relates to improvements in the so-called plasma spray coating technique, in which materials such as metals, ceramics, etc. are molten by means of a heavy current flowing through gas, i.e., the so-called arc or high-temperature plasma generated by the heavy current, and they are sprayed against an object to be treated for forming a strong coating film on its surface.
2. DESCRIPTION OF THE PRIOR ART
A heretofore known plasma spray coating apparatus is the apparatus illustrated in FIG. 16, in which a cathode 1 of the apparatus is held concentrically with an anode nozzle by means of an insulator 12 so that a tip end of the cathode 1 may come to the proximity of an inlet of a nozzle pipe passageway 25 of the anode nozzle, and at the upstream of the tip end, plasma gas 8 is charged through a plasma gas charging port 7. A negative side of a power source 3 is connected to the cathode 1 by a lead 5, and a positive side of the power source 3 is connected to the anode nozzle 2 through an exiting power source 4 by a lead 6. It is to be noted that reference numeral 13 designates a cooling system, the interior of the anode nozzle 2 is normally constructed in a double structure, though not shown, and the interior is adapted to be continuously cooled by softened coolant water or the like through pipings 14 and 15. Now, if a high-frequency voltage is applied by an exciting high-frequency power source 4 between the cathode and the anode while a D.C. voltage is applied therebetween by a power source 3 and as making plasma gas, normally inert gas such as argon shown by arrows 8 and 9 flow through the anode nozzle 2, then arc is generated from the tip end of the cathode 1 towards an inner surface 105 of the nozzle pipe passageway 25 of the anode nozzle 2. Since such short arc is apt to damage an inner wall of the nozzle pipe passageway 25 of the anode nozzle 2, i.e., a nozzle pipe wall 26, a large amount of plasma gas 8 is made to flow so that arc 11 may be formed within the nozzle pipe passageway 25 over a distance as long as possible to form an anode point 10 far from the tip end of the cathode 1. The plasma gas flowing through the nozzle pipe passageway 25 of the anode nozzle 2 is strongly heated up to a high temperature by the thus formed arc 11, it takes a state of the so-called plasma flame 16 and is ejected from the tip end of the anode nozzle. At this moment, if spray coating material 18 is charged through a material charging pipe 17, then the material is mixed in the plasma flame 16 at a high temperature ejected from the anode nozzle 2, and momentarily converted to molten material 20 to be sprayed onto an object to be treated, i.e., a substrate 22, and thereby a coating film 21 is formed on the surface of the substrate. It is to be noted that in a certain case the spray coating material 18 is fed to just behind an outlet of the anode nozzle 2 as illustrated by the material charging pipe 17, but in another case the material charging pipe is disposed just in front of the outlet of the anode nozzle 2 as shown by arrow 23. In either case, in this type of plasma spray coating apparatus used in the past, an extremely large amount of gas was used to form long arc 11 within the anode nozzle 2 for preventing corrosion of the inner wall 26 of the anode nozzle 2 and to cool the nozzle pipe wall 26 of the anode nozzle 2 by means of the plasma gas 8 and 9, the ejecting speed of the plasma flame 16 at the tip end of the anode nozzle 2 was normally maintained at an extremely high speed condition in the range of Mach 0.5 to Mach 3, consequently, in the spray coating apparatus in the prior art, very violent noises of the order of 110 phons to 120 phons were generated, and therefore, the plasma spray coating apparatus had a great disadvantage that normally the operation of the apparatus was possible only within an isolated soundproof room, and an operator for operating the apparatus could not be in charge of manipulation for operating the apparatus unless he wore a noise protecting device. Furthermore, since the plasma gas ejected from the tip end of the anode nozzle 2 normally forms a violent brilliant flame containing a large amount of ultraviolet rays, it is impossible to directly look at the flame, and so, an operator of the apparatus is compelled to wear ultraviolet rays protecting glasses. In addition, for the plasma gas used in the spray coating apparatus in the prior art, normally expensive inert gases such as argon, helium, hydrogen, etc. are used. This is because if gases having a strong activity such as air, oxygen or the like are used as the plasma gas, the nozzle pipe wall 26 is quickly oxidized and worn and continuous operation for a long period becomes impossible. Since these inert gases are expensive and they are consumed in a large amount for the purpose of generating a high speed within the nozzle, there is also a large shortcoming that an extremely high operating cost is required. Moreover, in the plasma spray coating apparatus in the prior art, the plasma flame 16 ejected from the tip end of the apparatus has an extremely strong turbulent flow condition due to its remarkably high speed, and therefore, as shown by arrows 27 a large amount of atmospheric air in the proximity of the ejecting port is swirled and sucked, resulting in a quick lowering of the temperature of the plasma gas. Accordingly, in order to carry out spray coating under a proper condition, the distance between the tip end of the anode nozzle 2 and the substrate 22 is required to be maintained extremely precisely, if this is deviated it becomes very difficult to form a proper coating film, accordingly, extremely severe control for an operating condition is required for the purpose of quality control of the coating film, and so, quality control is not easy. In addition, in the heretofore known plasma spray coating apparatus, since an extremely large amount of high speed gas is violently sprayed towards the substrate 22 in view of the situation as described in detail above, the substrate 22 is limited to that having a high strength, and the apparatus is not suitable for micro-fine working. Also, the plasma spray coating apparatus in the prior art had a shortcoming that inert gas such as argon, helium, etc. is used as the plasma gas 8, and hence the cost of the plasma gas becomes high.